The invention relates to dynamoelectric machine commutator structures and more particularly to improvements in such structures whereby effective seals are provided at the junctions of commutator banding means and commutator conductor segments for the purpose of preventing the build-up of conductive contaminants in fissures formed between those members by thermocycling and other forces causing relative movement of those members during commutator operation.
At the present time it is generally well known in the manufacture of commutator structures for dynamoelectric machines to utilize pre-stressed glass bands, or other suitable commutator banding materials, mounted in pre-formed annular grooves on a commutator surface to provide an effective means for securing conductive commutator segments rigidly in position against the relatively high centrifugal forces exerted on them during rotation of the commutator. For example, U.S. Pat. No. 4,170,505 which issued on Oct. 9, 1979 and is assigned to the assignee of the present invention, discloses a glass banded commutator structure and a method for making it by utilizing an irradiation curable resin to lock glass bandings in operating position on the commutator.
It was also known well before the present invention that conductive commutator segments could be adhesively bonded successfully to a supporting hub or base member. Such an adhesively bonded commutator structure is shown in U.S. Pat. No. 3,751,700, which issued Aug. 7, 1973 and is also assigned to the assignee of the present invention. In practicing the types of commutator binding or banding operations described in the foregoing two patents as well as those otherwise generally in use at the present time, a variety of different banding materials have been proven suitable for such applications. For example, as mentioned in U.S. Pat. No. 3,146,364 which issued Aug. 25, 1964, a commutator banding or tape may be suitably formed of resin-impregnated roving comprising fiberglass or other non-conducting fibers such as those sold under the trade names "Nylon" and "Dacron". As is illustrated by the methods described in each of the foregoing patents, the more recent commutator banding methods known in the prior art typically require a resin impregnated banding tape or roving to be wound directly into a suitable annular channel or groove in the outer circumferential surface of a commutator. The ease with which such bands are quickly and effectively positioned on a commutator, coupled with the effectiveness of those bands in rigidly securing the commutator segments in operating position against the forces tending to move the commutator bars when the commutator is rotated at high speed, have caused such banding techniques to be widely adopted. Moreover, because the banding tapes and associated resins are usually both dielectric in nature, it was not, prior to the present invention, believed to be either necessary or desirable to provide any particular insulation or other coatings between the commutator bands and the commutator conductive segments.
However, the inventors of the invention described herein have discovered that during the normal operation of a commutator that is banded with known prior art methods, such as those described in the above mentioned patents, it is possible in frequently encountered commutator operating conditions for electrically conductive contaminants to be deposited in cracks and fissures between the commutator bands and the adjacent commutator conductive segments. In the event that a sufficient thickness of conductive contaminants is so deposited, it is possible that electrical tracking will occur over the deposits, between adjacent conductor segments, thereby resulting in carbon buildup that eventually creates a sufficient current path to short circuit the commutator.
Normal thermal cycling and mechanical forces applied to a commutator as a consequence of its operation may be sufficient to create enough relative movement between commutator bands and associated commutator segments, to cause cracks and fissures at the junction of these members. Once such fissures are formed, they readily collect deposits of carbon dust and other electrically conductive contaminants that are often present in the operating ambient. Many of these deposits can lead to the undesirable type of commutator bar shorting just described. In addition, and in the absence of such fissures, it is possible that adjacent commutator segments can be short circuited due to the buildup of moisture or carbon dust on the surface of a commutator banding material. Such undesirable buildup of contaminants on the banding surfaces can easily bridge the mica or other insulating materials that are disposed between adjacent commutator conductor segments, thereby producing a tracking or arcing path which causes further carbonization of the underlying bonding resin in the glass bands. That kind of carbonization will eventually short circuit the commutator segments as well as cause degradation of the banding material to such an extent that it may fail to perform its desired clamping function.